1. Field of the Invention
The present invention relates to the field of drying agents for waterborne paints and more particularly to porous silica drying agents for rapidly drying waterborne latex paint compositions.
2. Description of the Related Art
Two desirable features for highway marking paint compositions are that the compositions are durable and that they dry rapidly. Because of the manpower involved in marking roadways and the associated material costs, it is desirable that a highway marking last a minimum of 4 years and more preferably up to 7 years or longer. Moreover, it is highly desirable that the highway marking dry rapidly to prevent the paint from running across the roadway and to minimize the disruption to traffic flow while the highway marking is being applied.
One technique to enhance durability is to apply a thicker layer of paint. A thin paint layer of 25 mil or less will likely have a 1 year life and at best 2-3 years. A thicker paint layer of 60 mil or greater will typically last for 4-7 years. Accordingly, there is a strong desire to apply a thicker paint layer when marking highways.
The main constraint to applying a thick highway paint layer is the drying time of the paint. Thermoplastic materials that are heated immediately after application to set the paint layer have regularly been used to obtain the desired thick highway paint layer. However, because of its cost and need for extra machinery, there has been an effort to develop a less expensive paint system. Several of these systems are discussed below.
U.S. Pat. No. 6,013,721 describes the use of polymer containing one acid and one amine functionality to improve the storage stability and drying time of the waterborne paint. The system was able to produce a paint layer having a wet film thickness of 14 mil.
U.S. Pat. No.5,544,972 shows the use of spraying mineral acid into paint stream to accelerate the drying time of binders. This acid spray when comes to contact with paint coagulates the resin salt by an acid-base reaction. This method is not practical in roads as use of mineral acids pose a lot health and other problems.
U.S. Pat. No. 5,947,632 describes the use of ion exchange resins as drying agents. Ion exchange resins are hollow polymers which absorb water in the paint and reduce the drying time on traffic markings. Such ion exchange resins include super absorbent polymeric gels (Sumica gel) and solid hollow sphere polymers (Ropaque® OP-62). This patent also mentions inorganic compounds capable of absorbing water by coordination, although experimental examples are provided only for the ion exchange resins.
U.S. Pat. No 6,132,132 discloses various types of drying agents including ion exchange resins, superabsorbent gels such as Sumica gel, and hollow-sphere polymers such as Ropaque® OP-62 to improve the drying time of the waterborne paint for traffic markings.
U.S. Pat. No. 6,475,556 discusses the need for fast drying of thick films of waterborne paint above 20 mil and up to 120 mil in thickness. However methods are disclosed that only dry films of 20-40 mil wet thickness. An ion exchange resin is used as the drying agent and a small amount of polyamine resin is added to the binder as well.
U.S. Pat. No. 6,413,011 describes multicomponent fast drying waterborne coatings of 13-15 mil wet film thickness. These coatings include extenders such as calcium carbonate, talc, silica and silicates as well as fillers such as glass beads, quartz and sand. The disclosed extenders are hard dense materials.
U.S. Pat. No. 6,645,552 describes the use of ion exchange resin as drying agent for waterborne paint with wet mil thickness of 30 mil. The paint is applied as a wet on wet or layer by layer coating; with each layer being 15 mil.
U.S. Pat. Nos. 6,013,721, 5,947,632, 6,132,132, 6,475,556, 6,645,552 and 6,413,011 all describe the use of large particle silica gel (Silica S21), alumina, silica-aluminas, silicas and silicates as extenders in paint formulations. Extenders are inactive materials present in any paint formulation to improve color and flow properties of the paint.
U.S. Pat. No. 5,340,870 describes the use of a filler such as calcium carbonate added to an acrylic binder to increase the viscosity and dry time of the paint. The filler is added at a 60-75% ratio relative to the binder. Because of the high content of the inorganic filler the paint dries fast but its storage stability is poor and it does not give durable markings once applied on roads.
U.S. Pat. No. 6,333,068 describes the use of metallic polychloride and synthetic polyamides as accelerators. These accelerators dry the 15 mil thickness waterborne paint fast.
U.S. Pat. No. 5,922,398 describes the use of pendant amino-containing polymer along with acrylic polymers so that the pH is adjusted to provide for fast drying. The coatings achieved with this system are described as having a wet film thickness of 15 mil.
Although the systems discussed in the patents above were able to speed up drying time of the highway paint, they were not able to achieve waterborne traffic markings having a thickness greater than 30 mil wet thickness. At such a thickness, the underlying roadway will need to be repainted on an annual basis. Moreover, the ion exchange resins described above absorb water at the same time the sulfonic acid group slowly reacts with acrylic resins thus weakening the resin. In addition, the brown color of the ion exchange resin makes the white highway marking line dirty and brown spots are seen after the traffic lines are striped. There is a need for a waterborne traffic marking system which has greater thickness and increased durability and will not discolor the marking stripe.